1. Technical Field of the Invention
The present invention relates to a gas-turbine blade provided with a steam-cooled structure.
2. Related Art
In recent years, it has been thought of to use steam in place of air for cooling the blades of a gas turbine in a combined cycle power plant, and to recover the steam used for cooling the blades without discharging it into a main gas flow with a view to improving the thermal efficiency of the gas turbine, (see, for example, Japanese Patent Application Laid-open No. 8-319803)). However, such approach has not yet found practical application.
With such steam cooling system, heat energy from the gas turbine carried by the recovered steam can be utilized in a steam turbine, whereby efficiency of the plant on the whole can be protected against degradation. Further, by suppressing the amount of cooling medium or coolant fed to the gas turbine, turbine efficiency can be enhanced. Additionally, by using steam as the coolant instead of air, heat transfer performance can be significantly enhanced without the need for changing or altering the geometrical configuration of the existing coolant flow passages.
A typical internal cooling structure of a moving blade in a conventional heat recovery type steam-cooled gas turbine, such as mentioned above, is shown in FIGS. 5a and 5b. Moreover, FIG. 5a is a vertical section of a blade, and FIG. 5b is a sectional view of same along the line 5B--5B in FIG. 5a.
Steam for cooling the moving blade 1 is supplied through a cooling steam inlet port 8 provided in a lower end portion of the blade at a location close to a leading edge 5 of the blade, and the steam flows through a coolant flow passage 4 formed inside the moving blade 1 in a serpentine pattern, as indicated by the arrows. After having cooled the interior of the blade, the steam leaves the blade through a cooling steam outlet port 9 provided at a location close to the blade trailing edge 6 and is subsequently introduced into a recovery system not shown.
Further, a plurality of turbulence promoting fins 7 are formed on the inner surfaces of the coolant flow passage 4 in the blade, each extending in a direction substantially orthogonal to the flow of the coolant steam so as to promote internal heat transfer.
As mentioned previously, the coolant steam is recovered by equipment provided at a location downstream of the gas turbine. To this end, the pressure of the coolant steam within the blade is maintained higher than the pressure of gases flowing outside of the blade by, 2 to 4 MPa. Hence, the blade is subjected to internal pressures which may exceed a permissible limit predetermined by the strength of the hollow blade with a thin structure, thus involving deformation (bulging) of the blade and hence fluid delamination of the working gas flowing along the external surface of the blade, to incur such problems as degradation in the performance of the blade and the like. Thus, there exists a demand for a blade with a structure which can at least withstand the internal pressure mentioned above.